I will be doing video editing 1080p to 4k on final cut pro but occasionally,
The imac will be used for browsing,playing games fortnite etc and kids school work including photo editing with LR or similar programme.
My budget is around 1 k
The kids currently use a 21.5 late 2013 imac i5 2.9 ghz quad core 8gb ram nvidia g force GT 750m 1 gb
( its an old imac but still does the job but gets a bit alow)
If you're looking to go from a 2013 to a late 2015 you may not see dramatic improvements in performance, although you will definitely have a screen upgrade.
You mentioned a late 2015 system with the m390 graphics card. As I had mentioned, I also have a late 2015 27" iMac, but I maxed out the graphics card (m395X). I've compared performance in Capture One (Lightroom alternative) to someone with a similar system but with the lower-end graphics card, and the differences were stark. For example, they experienced lag when adjusting images, whereas mine was entirely smooth. That's a huge user experience difference, and tangible time impact.
My two takes:
1) If you're going to spend the money to upgrade, make sure it'll really count. In terms of processor architecture, the jump from a 2013 system to a 2015 system is only one generation. That's still something, but you'd experience a more impressive boost by going with a 2017 (two generations) or newer system.
2) Spend money on the parts you will be stuck with, and upgrade the rest later. You can upgrade your own RAM, so unless you're offered a good deal to have it all, take a system with the minimum amount of RAM and upgrade it later. An internal SSD would improve performance, but you can get away with a smaller size and plan to make up storage space with external drives (whether external SSDs or hard drives). If you get a 2017 system or newer you can get an external GPU, otherwise get the best GPU you can because there's no going back. Having to add on all of these options and run them externally has the potential to become expensive and somewhat ruins the aesthetic Apple was going for with the iMac, but it'd extend the life and usefulness of your system.